Thin film transistors (TFTs) using amorphous silicon or polysilicon have been put to practical use as drive elements for liquid crystal displays. Generally, these TFTs are formed in a manufacturing process in which various methods such as CVD, sputtering and photolithography are used in combination. Vacuum film-forming apparatuses used for the CVD and sputtering methods and photolithography apparatuses are both expensive, and thus the cost of these apparatuses accounts for a significant part of the manufacturing cost of liquid crystal panels. In addition, amorphous silicon films and polysilicon films are fabricated at high temperatures, and therefore substrates having low heat resistance, such as a plastic substrate, cannot be used for such films.
As a method of forming a TFT on a plastic substrate, for example, a method of forming a semiconductor layer using semiconductor nanomaterials such as Si nanowires and ZnO nanorods has been proposed (JP 2005-244240 A).
As another example, a method of forming an n-type semiconductor film having a mobility of approximately 0.1 cm2/V·s by using ZnO fine particles modified with organic molecules has been proposed (Electron Devices Meeting, 2004, IEDM Technical Digest. IEEE International, 13-15 Dec. 2004, pages 769-772, “A novel transparent air-stable printable n-type semiconductor technology using ZnO nanoparticles”). In this method, baking is carried out after the ZnO fine particles are applied, and the baked surface further is subjected to hydrogenation.
A TFT using an amorphous InGaO3.ZnO semiconductor film also has been disclosed (Nature, Vol. 432, pages 488-492, November 2004, “Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors”). The amorphous InGaO3.ZnO semiconductor film is formed at room temperature by a pulse laser ablation method, which is one of the vacuum film-forming methods. The amorphous film that is formed by adding indium and gallium into zinc oxide has a higher mobility than that of an amorphous silicon film. Furthermore, single crystalline InGaO3(ZnO)m complex oxide has been fabricated by a unique technique (Reactive Solid Phase Epitaxy) in which ZnO and InGaO3 thin films formed by pulse laser ablation are covered with an oxide substrate and are subjected to annealing at 1400° C. in a sealed condition (Thin Solid Films, Vol. 445, pages 322-326, 2003, “Electron transport in InGaO3(ZnO)m (m is an integer) studied using single-crystalline thin films and transparent MISFETs”).
Semiconductor nanomaterials such as semiconductor nanowires and semiconductor nanorods alone have high mobility. However, these semiconductor nanomaterials are strongly anisotropic in their properties. Therefore, in order to form a semiconductor film having high mobility by using such semiconductor nanomaterials, they need to be oriented in a specific direction. However, it is not easy, in the state of the art, to orient the nanomaterials.
On the other hand, it is possible to fabricate semiconductor films using ZnO fine particles or CdSe fine particles by an easy process. However, these semiconductor films have lower mobilities than that of an amorphous silicon film, which is not satisfactory in their performance to drive liquid crystal elements and organic EL elements.
Semiconductor films using semiconductor nanomaterials or semiconductor fine particles have an advantage that they can be formed at low cost because they can be formed without a vacuum film-forming apparatus. These semiconductor films have another advantage that they can be formed by a low-temperature process. Therefore, TFTs using semiconductor nanomaterials or semiconductor fine particles have been studied for more than 10 years as promising TFTs. However, these TFTs have not yet achieved successfully both the ease of the manufacturing process and the satisfactory performance, and thus have not yet been put to practical use.